From a virustotal blog, " Deleting the option Do not Distribute": »
blog.hispasec.com/virust ··· t:int=20said by Virustotal :When we launched VirusTotal back in 2004, the non-distribution option was intended to allow the analysis of files and documents containing sensitive data with the complete certainty they would not be sent to AV labs at all. Until now, the main use of this option has been the aforementioned: Analyzing Word files, PowerPoint presentations, PDF files, etc., that contained sensitive data.
Besides this initial function, afterwards we realized other alternative uses could be applied, by both, computer security professionals and malware specialists, as well as malware developers.
Just be aware that if you send a file with sensitive information to Virustotal that is detected by at least one AV it will be distributed to the other participants. Probably not a good idea to scan your porn collection either.

Malware developers get around this by using underground scanners, cracked scanners, and installing free version scanners in virtual machines.